It's the "Passes" variant of the Unknown maps.
The issue can be observed vey well with the pathfinder grid in atlas (#4842).
(In this variant there is a small disk shaped area in the middle, acting as a filler for any part of the central area that wasn't filled by the mountainrange.
So first thought was to increase the size, but the bug still occurs when increasing the radius by a factor of 3.)
The issue occurs exactly when the waveforms of two neighboring mountains coincidentally result in them being nearly parallel. Only then the mountains become reachable after the smoothing of the elevation with neighboring tiles.
Reducing the smoothing from 3 to 1 solves the issue described in this report, (but not the nomad mountain bug reported in #3140)
